Although the fibers of the present invention are believed suitable for reinforcing a number of matrix materials, such as adhesives, asphalt, composites, plastic, rubber, etc. and structures made therefrom, they are primarily intended for reinforcing hydratable cementitious compositions such as ready-mix concrete, precast concrete, masonry concrete, shotcrete, bituminous concrete, gypsum compositions, gypsum- and/or Portland cement-based fireproofing compositions, and other hydratable cementitious compositions. A major purpose of the fibers of the present invention is reinforcing concrete (e.g., ready-mix, shotcrete, etc.) and structures made from these. The task of reinforcing matrix materials such as these poses one of the greatest challenge for designers of reinforcing fibers.
Concrete is made using a hydratable cement binder, a fine aggregate (e.g., sand), and a coarse aggregate (e.g., small stones, gravel), and is consequently a brittle material. If a concrete structure is subjected to stresses that exceed its maximum tensile strength, then cracks can be initiated and propagated in the concrete. The ability of a concrete structure to resist crack initiation and crack propagation can be understood with reference to the "strength" and "fracture toughness" of the fibers.
Fiber "strength" relates to the ability of a cement or concrete structure to resist crack initiation. In other words, fiber strength is proportional to the maximum load sustainable by the structure without cracking, and is a measurement of the minimum load or stress (e.g., the "critical stress intensity factor") required to initiate cracking in that structure.
On the other hand, "fracture toughness" relates to the specific "fracture energy" of a cement or concrete structure. This concept refers to the ability of the structure to resist propagation--or widening--of an existing crack in the structure. This toughness property is proportional to the energy required to propagate or widen the crack (or cracks). This property can be determined by simultaneously measuring the load required to deform or "deflect" a fiber-containing concrete (FRC) sample at an opened crack and also measuring the amount or extent of deflection. The fracture toughness is therefore determined by dividing the area under a load deflection curve (generated from plotting the load against deflection of the FRC specimen) by its cross-sectional area.
In the cement and concrete arts, fibers have been designed to increase the strength and fracture toughness in reinforcing fibers. Numerous fiber materials can be used for these purposes, such as steel, synthetic polymers (e.g., polyolefins), carbon, nylon, aramid, and glass. The use of steel fibers for reinforcing concrete structures remains popular due to the inherent strength of the material. However, one of the concerns in steel fiber product design is to increase their "pull out" resistance because this increases the ability of the fiber to defeat crack propagation. In this connection, U.S. Pat. No. 3,953,953 of Marsden disclosed fibers having "J"-shaped ends for resisting pull-out from concrete. However, stiff fibers having physical deformities may cause entanglement problems that render the fibers difficult to handle and to disperse uniformly within a wet concrete mix. More recent designs, involving the use of "crimped" or "wave-like" polymer fibers, may have similar complications, depending on the stiffness of the fiber material employed.
U.S. Pat. No. 4,414,030 of Restrepo disclosed the use of microfibrillated polyolefin filaments that are oriented in all spatial directions by subjecting fibrillated ribbons to air, thereby spreading out the separate fibers, and then feeding these separated fibers into a mortar mixing machine fitted with a high-speed propeller to blend the mortar components and fibrous materials together. The mechanical shredding action which takes place in the mixing operation causes the ribbons to become further fibrillated, such that the ribbon fibrils are broken apart into individual filaments having a branched structure with microfibrils outwardly projecting along their length. The projected microfibrils are somewhat curled in shape and perform as anchoring elements or "hooks" within the cement hardened matrix. It is generally believed that side branches or "hooks" can act to resist fiber dislodgment or pull-out from the cement matrix and present enlarged surface area for anchoring within concrete. The physical branched fiber structure would appear to create entanglement problems that would render handling and dispersion within a wet concrete mix somewhat difficult to achieve.
U.S. Pat. No. 5,753,368 of Berke et al. taught fibers having a glycol ether-based coating for enhancing bond strength of the fibers within concrete. Berke et al. further taught that the fibers could be bundled using mechanical or chemical means, and that the fibers could be introduced into a cement composition using packaging technology to facilitate mixing and dispersion within concrete. This technology may be applied to varieties of fibers and shapes to enhance pull out resistance while facilitating uniform dispersion within the concrete mix.
U.S. Pat. No. 5,298,071 of Vondran discussed the problem of achieving a uniform dispersal of fibers within a wet cement mix. Vondran noted that fibers were typically added to the mixer with the cement, sand, aggregate, other admixtures, and water. His approach was to add fiber precursors (e.g., steel fibers and polyolefin in the form of extruded monofilament or fibrillated sheet fiber) and cement clinker to a ball mill grinder and to obtain a hydratable mixture comprising interground fibers in a dry hydratable cement powder that could then be used for making the concrete structure.
It is readily observed that Vondran's clinker/fiber-intergrinding method (hereinafter the "Vondran method") purports to achieve quick fiber wetting and uniform dispersion without the balling and clumping found when adding the fiber components separately into concrete. The present inventors, however, observe that the Vondran method teaches that "fiber precursors" are combined with cement clinker particles into a ball mill cement grinder, and that this process provides fibers that are "attenuated, roughened and abraded by the action of the clinker particles and the grinding elements on the fiber" (See U.S. Pat. No. 5,298,071 at column 2, lines 58-66). This process purportedly results in improved mechanical bonding between the cement and fibers.
In the present invention, however, the inventors seek to improve the pull-out resistance of fibers from concrete while avoiding the kinds of mechanical or physical fiber attributes that might otherwise impede the ability of the fiber to be introduced into, and uniformly dispersed within, the concrete mix. The present inventors believe that the clinker intergrinding process of Vondran results in cement particles being ground into, and embedded in, the fiber surface. Moreover, the deep-abrading action of the cement clinker may be undesirable because the fibers will tend to clump during humid conditions (e.g., storage, shipment) due to the hydrating cement particles. Furthermore, fibers can not be interground with clinker at high volumes using ball mill machinery in an clinker-intergrinding process because the fibers would potentially clog the classifier unit used in such mills for separating ground cement particles from the grinding operation. The present inventors have also discovered that fibers interground in ball mill operations using clinker are severely abraded, and, in effect, are shredded to the point at which their mechanical integrity, for purposes of reinforcing concrete, is defeated. Such clinker-interground fibers, whether by abrasion and/or impact of clinker material, lose mechanical resistance to pull-out from concrete (i.e., fracture toughness) because the fiber bodies and ends are shredded or devastated by the clinker/fiber intergrinding operation.
The terms "shredded" or "shredding" are used herein to refer to the tearing-apart of the fiber body into smaller elongated pieces. The concept of "shredding" as used herein is not equated herein with the concept of "fibrillation". The concept of fibrillation may be seen to occur where a multifilament fiber, comprised of two or more strands or fibrils are adhered or bonded together, is separated into its component strands or fibrils. On the other hand, "shredding" is defined for present purposes as the act of breaking a fiber down (whether monofilament or multifilament) into pieces smaller than the constituent strands or fibrils.
In view of the disadvantages of the prior art as discussed above, the present inventors believe that a novel fiber for reinforcing matrix materials, and in particular hydratable cementitious materials such as concrete and shotcrete, are needed. Also needed are novel methods for making such fibers and for modifying such matrix materials.